Method and system for performing die attach using a flame

ABSTRACT

Embodiments of a method for attaching a die to a substrate using a flame or other heat source are disclosed. The flame may be produced by combustible gas. Also disclosed are embodiments of a system for performing die attach using a flame. Other embodiments are described and claimed.

FIELD OF THE INVENTION

The disclosed embodiments relate generally to the packaging ofsemiconductor devices and, more particularly, to a method and system forattaching a die to a substrate using a flame or other heat source.

BACKGROUND OF THE INVENTION

An integrated circuit (IC) die may be attached, both mechanically andelectrically, to a package substrate. The IC die may have an array ofbond pads on the die's “front” side, and a solder bump or other lead maybe affixed to each of these bond pads. A mating array of lands isdisposed on the package substrate, and the die is placed face down onthe substrate such that the array of solder bumps extending from the dieare aligned with the mating array of lands on the substrate. The solderbumps extending from the IC die are then coupled to their respectivelands on the substrate. The package substrate may include multiplelayers of conductors (e.g., traces), and these conductors can routeelectrical signals (running to and from the die) to locations on thepackage substrate where electrical connections can be established with anext-level component (e.g., a motherboard, a computer system, a circuitboard, another IC device, etc.). For example, the substrate circuitrymay route all signal lines to a ball-grid array (or, alternatively, apin-grid array) formed on a lower surface of the package substrate, andthis ball- or pin-grid array then electrically couples the packaged ICdie to the next-level component, which includes a mating array ofterminals (e.g., lands, pin sockets, etc.). The use of an array ofsolder bumps (or columns, etc.) to couple an IC die to a substrate isoften referred to as Controlled Collapse Chip Connect (or C4).

As noted above, an array of solder bumps extends from the front side ofthe IC die—each of these solder bumps being coupled with a bond pad onthe die—and these solder bumps are coupled with a mating array of landson the package substrate. To couple these solder bumps to the matingarray of substrate lands, the assembly (e.g., die and substrate) may beplaced in an oven and heated to reflow the solder bumps. For lead-basedsolder compositions, the reflow temperature may be approximately 225degrees Celsius, and for lead-free solder compositions the reflowtemperature may be approximately 260 degrees Celsius. Uponsolidification of the solder bumps, an electrical and mechanical bond isformed between the solder bumps and their mating lands on the packagesubstrate.

During solder reflow, both the die and substrate may be heated to thereflow temperature, which can lead to thermal expansion of thesecomponents. However, the coefficient of thermal expansion (CTE) of theIC die may be substantially different than the CTE of the packagesubstrate. For example, a die made of silicon will have a CTE ofapproximately 3 ppm/° C., whereas an organic substrate may have a CTE ofapproximately 16 ppm/° C. Upon cool down after reflow, the reflowedsolder bumps solidify to form solid interconnects between the die andsubstrate. At the same time, due to the difference in CTE between the ICdie and package substrate, a differential thermal displacement occursbetween the die and substrate. Because of this differential thermaldisplacement, as well as the mechanical stiffness of the solidinterconnects that are formed, significant residual stresses maydevelop. These residual stress may impact both the IC die (e.g., thedie's interconnect structure) and the package substrate, as well as thesolder interconnects extending between these two components. Theseresidual stresses may, for example, result in die warpage as well ascracking of the die's interconnect structure. The IC die's interconnectstructure may be formed from a low-k dielectric material—which generallyhas lower mechanical strength in comparison to materials having a higherdielectric constant—and an interconnect structure formed from theselow-k dielectric materials may be especially sensitive to cracking as aresult of the above-described differential thermal displacement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an embodiment of a method ofattaching a die to a substrate using a flame or other heat source.

FIGS. 2A-2E are schematic diagrams illustrating further embodiments ofthe method shown in FIG. 1.

FIG. 3 is a graph showing one embodiment of the relationship betweenabsorbed heat flux and pulse duration.

FIG. 4 is a schematic diagram illustrating one embodiment of a systemfor attaching a die to a substrate using a flame or other heat source.

DETAILED DESCRIPTION OF THE INVENTION

Illustrated in FIG. 1 is an embodiment of a method 100 of attaching anintegrated circuit (IC) die to a substrate using a flame or other heatsource. Embodiments of the method 100 of FIG. 1 are further illustratedin FIGS. 2A through 2E, and reference should be made to these figures ascalled out in the text below.

Referring to block 110 in FIG. 1, an IC die is placed on a substrate.This is illustrated in FIG. 2A, which shows a die 220 that has beenplaced on a substrate 210. Substrate 210 may comprise a packagesubstrate or other die carrier, a circuit board, or any other suitableboard or substrate. In one embodiment, the substrate 210 comprises amultilayer substrate including a number of alternating layers ofmetallization and dielectric material. Each layer of metallizationcomprises a number of conductors (e.g., traces), and these conductorsmay comprise any suitable conductive material, such as copper. Further,each metal layer is separated from adjacent metal layers by thedielectric layers, and adjacent metal layers may be electricallyinterconnected by conductive vias. The dielectric layers may compriseany suitable insulating material—e.g., polymers, including boththermoplastic and thermosetting resins or epoxies, ceramics, etc.—andthe alternating layers of metal and dielectric material may be built-upover a core layer of a dielectric material.

The die 220 may comprise any type of integrated circuit device. In oneembodiment, the die 220 comprises a microprocessor, a network processor,an application specific integrated circuit (ASIC), a field programmablegate array (FPGA), or other processing system or device. It should,however, be understood that the disclosed embodiments are not limited tothe aforementioned processing devices and, further, that the die 220 maycomprise any other type of device (e.g., a wireless communicationdevice, a chip set, a memory device, etc.).

Die 220 includes a “front” side 221 and an opposing “back” side 222. Asthe reader will appreciate, the labels “front” side and “back” side arearbitrary, and the opposing surfaces 221, 222 of die 220 may bereferenced by any other suitable terminology or nomenclature. The die220 may, in one embodiment, have a layer of metal (or other material)disposed on the back side 222. However, in another embodiment the dieback side 222 has no back side metallization. For example, the back sidesurface may be in the “as is” condition after IC fabrication (e.g., apolished surface).

According to one embodiment, an array of conductive bumps 230 or otherconductive leads extends from the front side 221 of die 220, each of theconductive bumps being electrically connected to a bond pad (not shownin figures) on the die. In one embodiment, the conductive bumps 230comprise solder; however, in other embodiments the conductive bumps maycomprise other materials (e.g., copper, aluminum, etc.). The array ofconductive bumps 230 mates with a corresponding array of conductivelands (not shown in figures) formed on an upper surface 211 of thesubstrate 210. When the conductive bumps 230 are connected with theirrespective lands on substrate 210 (e.g., by a reflow process, asdescribed below), electrical communication can be established betweenthe die and substrate. It should be noted that, in other embodiments, anarray of conductive bumps (or other leads) may extend from the substrate210, and this array of leads mates with a corresponding array of bondpads on the die.

With reference to block 120 in FIG. 1, in one embodiment, a heat shieldis placed over and/or around the die. This is illustrated in FIGS. 2Band 2C, which shows a heat shield 250 that has been disposed around thedie 220. According to one embodiment, as shown in the figures, the heatshield 250 comprises a rectangular-shaped plate having an aperture 255that is sized and oriented to receive the die 220. As will be describedbelow, the back side 222 of die 220 will be subjected to a flame (orother heat source) during a die-attach process, and the heat shield may,in one embodiment, minimize heat transfer to the underlying substrate210. Thus, the heat shield 250 may function to reduce thermal expansionof the substrate 210—and, accordingly, differential thermal expansionbetween the die 220 and the substrate—as well as to protect thesubstrate 210 from excessive heat and/or heat-induced damage (e.g.,charring, etc.). In yet another embodiment, as set forth in block 125,the aperture 255 in heat shield 250 functions to align the die 220relative to substrate 210. Heat shield 250 may comprise any suitablematerial (or combination of materials), such as copper, aluminum,stainless steel, or a ceramic material. It should be understood that theheat shield 250 shown in the figures is but one embodiment of such adevice and, further, that the heat shield may comprise any suitabledevice, may be of any suitable shape or configuration, and may beconstructed from any suitable materials or combination of materials.

Referring now to block 130 in FIG. 1, aflame or other heat source isapplied to the back side of the die to initiate reflow or bonding of thedie leads (and/or reflow or bonding of leads extending from the packagesubstrate). This is illustrated in FIG. 2D, where a flame 275originating from a source 270 is applied to the back side 222 of die220. The flame 275 may, in one embodiment, extend down to the die backside 222. In another embodiment, the flame 275 is in close proximity tothe die back side 222. According to one embodiment, the flameencompasses substantially all of the die back side 222, whereas in afurther embodiment the flame encompasses a portion of the die back side222 and/or is targeted at a specific region of the die back side. In oneembodiment, the flame is directed at the die 220, and a substantialportion of the heat shield is not directly subjected to the flame(although portions of the heat shield proximate the aperture 255 may bedirectly subjected to the flame 275), which may result in localizedheating of the die. In other embodiments, however, the flame 275 mayextend down to and over a substantial portion of the heat shield 250.

The flame 275 may be produced by any suitable source 270. For example,the flame may be produced by combustion of acetylene, butane, propane,MAPP (methylacetylene propadiene), or other combustible gas. However, inother embodiments, the flame is produced by a combustible liquid (e.g.,kerosene) or a combustible solid material. It should also be understoodthat the disclosed embodiments are not limited to a heat source thatproduces a flame. For example, in other embodiments, the heat source maycomprise super heated air, plasma, or steam.

In one embodiment, the flame 275 (or other heat source) should heat thedie 220 to a temperature sufficient to initiate reflow of the solderbumps 230 (or, more generally, to initiate bonding of the die leads tothe substrate). According to one embodiment, as a result of heating byflame 275, the temperature at the die front side 221 reachesapproximately 225 degrees Celsius, or greater. In a further embodiment,the temperature at the die front side 221 does not exceed approximately415 degrees Celsius during reflow.

As noted above, the flame 275 (or, more generally, the supplied energy)may be localized at the die 220, which can help to minimize heattransfer to the substrate 210 (and heat shield 250). Heat transfer to,and heating of, the substrate 210 can also be reduced by minimizing theduration of the applied flame (or other energy pulse). Referring to FIG.3, illustrated is an exemplary embodiment of a relationship betweenabsorbed heat flux (at the die back surface 222) and heat pulseduration. As can be observed from this graph, as the pulse durationdecreases, the absorbed heat flux needed at the die surface willincrease. However, although the absorbed heat flux increases, heattransfer to the substrate is still minimized due to the short pulseduration. In one embodiment, the flame 275 is applied for a pulseduration of approximately 2 seconds, or less. In another embodiment, theabsorbed heat flux at the die back side 222 is approximately 40 W/cm²,or greater. In a further embodiment, the absorbed heat flux (andincident heat flux) spatially varies across the die back surface 222.For example, the absorbed heat flux may be greatest proximate the die'speriphery and decrease toward the center of the die. It should be notedthat the thermal efficiency of heat transfer from the flame to the diewill likely not achieve 100 percent—and, in practice, may be much lessthan 100 percent—and the incident heat flux needed at the die backsurface 222 may be significantly greater than the absorbed heat flux.Also, as the reader will appreciate, the absorbed heat flux and pulseduration will be dependent upon the die material, die thickness, and diesize, as well as other factors, and the actual values of these and otherparameters will vary on a case-by-case basis. Thus, no unnecessarylimitations should be drawn from FIG. 3 and the examples set forthabove.

According to one embodiment, while the die 220 is heated to atemperature sufficient to initiate reflow or bonding, the substrate 210is not substantially heated (and/or only minimal portions of thesubstrate 210 are substantially heated). Reduced heating of thesubstrate 210 results, at least in part, from the use of heat shield250, from the localized heating of the die 220, and/or from the rapidheating (e.g., short pulse duration) of the die. Because the substrate210 is not substantially heated, the thermal expansion of this componentduring die attach is significantly reduced (or, perhaps, eliminated).Thus, the residual stresses—and the resultant damage, such as diecracking or warpage—that can be caused by differential thermaldisplacement between the die and substrate is minimized.

After reflow and solidification of the solder bump leads 230 (or, moregenerally, after bonding of the die leads to the substrate), the die 220is both mechanically and electrically attached to the substrate 210.This is illustrated in FIG. 2E, which shows an assembly 200 includingthe die 220 and substrate 210, wherein the reflowed solder bumps 230 nowform solid interconnects between the die and substrate. In oneembodiment, because the differential thermal displacement between thedie and substrate is minimized—and, hence, the resultant residualstresses reduced—no underfill material is disposed between the die andsubstrate. However, in other embodiments, an underfill material (notshown in figures) may be disposed between the die 220 and substrate 210.

Referring again to FIG. 1, and block 140 in particular, a cleaningprocess may be performed. The flame attach process may produceby-products, such as oxides and/or carbon- or hydrocarbon-containingsubstances, and a cleaning process may be employed to remove theseby-products. Any suitable cleaning process or cleaning solutions may beused to clean the assembly 200. In one embodiment, the assembly 200 maybe cleaned by rinsing the assembly using—or by immersing the assemblyin—a solvent or flux. For example, the by-products of the flame attachprocess may be cleaned using a solution including alcohol, acetone, orwater. Also, in some embodiments, a cool down period may be needed priorto cleaning.

Referring now to FIG. 4, illustrated is an embodiment of a system 400that may be used to implement the above-described die-attach process.The system 400 includes a device 405 (e.g., a tray, chuck, clamp, etc.)for holding a substrate 410. System 400 also includes a pick-and-placemachine 490. The pick-and-place machine may include a first tool 401 forholding and moving a die 420 (that is to be placed on substrate 410).Pick-and-place machine 490 may include a second tool 402 for holding andmoving a heat shield 450 (that is to be placed over and/or around thedie 420 during die attach). The heat shield 450 includes an aperture 455that is sized and oriented to receive the die 420. The pick-and-placemachine 490 may further include a third tool 403 for moving and/orsupplying a flame source 470 (or other heat source).

In operation, the pick-and-place machine 490 may position the die 420 onsubstrate 410, and this machine may also position the heat shield 450over and/or around the die 420. Either the die 420 or heat shield 450may first be placed over the substrate 410, and the aperture 455 in heatshield 450 may function to align the die 420 on the substrate. The flamesource 470 is also positioned by pick-and-place machine 490 at thedesired position relative to the die and substrate. The pick-and-placemachine may further be used to place the substrate 410 on holding device405. With the substrate 410, die 420, heat shield 450, and heat source470 appropriately positioned, attachment of the die 420 to substrate 410may be performed using a flame produced by source 470, as describedabove (see FIGS. 1-3 and the accompanying text).

The foregoing detailed description and accompanying drawings are onlyillustrative and not restrictive. They have been provided primarily fora clear and comprehensive understanding of the disclosed embodiments andno unnecessary limitations are to be understood therefrom. Numerousadditions, deletions, and modifications to the embodiments describedherein, as well as alternative arrangements, may be devised by thoseskilled in the art without departing from the spirit of the disclosedembodiments and the scope of the appended claims.

1. A method comprising applying a flame to one side of a die to reflow anumber of solder leads extending between an opposing side of the die anda substrate.
 2. The method of claim 1, wherein the flame is produced bycombustion of a gas.
 3. The method of claim 1, wherein the flame isapplied for a duration of up to approximately 2 seconds.
 4. The methodof claim 1, wherein an absorbed heat flux at the one side of the die isat least approximately 40 W/cm².
 5. The method of claim 1, wherein theflame extends to the one side of the die.
 6. The method of claim 1,wherein the flame extends to a location proximate the one side of thedie.
 7. The method of claim 1, further comprising placing a heat shieldaround the die.
 8. The method of claim 1, wherein the die front sidereaches a temperature in a range of between 225 degrees and 415 degreesCelsius.
 9. A method comprising: placing a die on a substrate, the diehaving a front side and an opposing back side, the die having a numberof leads extending from the front side, each of the leads mating with acorresponding one of a number of lands on the substrate; and applying aflame to the die back side to heat the die and bond each die lead to thecorresponding one land on the substrate.
 10. The method of claim 9,wherein each of the die leads comprises a solder bump.
 11. The method ofclaim 9, wherein the flame is produced by combustion of a gas.
 12. Themethod of claim 9, wherein the flame is applied for a duration of up toapproximately 2 seconds.
 13. The method of claim 9, wherein an absorbedheat flux at the die back side is at least approximately 40 W/cm². 14.The method of claim 9, wherein the flame extends to the die back side.15. The method of claim 9, wherein the flame extends to a locationproximate the die back side.
 16. The method of claim 9, furthercomprising placing a heat shield around the die.
 17. The method of claim9, wherein the die front side reaches a temperature in a range ofbetween 225 degrees and 415 degrees Celsius.
 18. A system comprising: adevice to hold a substrate; and a flame source to heat a die disposed onthe substrate and reflow a number of leads extending between the die andsubstrate.
 19. The system of claim 18, further comprising a device toplace the die on the substrate.
 20. The system of claim 18, furthercomprising a heat shield to position around the die.
 21. The system ofclaim 18, wherein the flame source comprises a combustible gas.
 22. Thesystem of claim 18, further comprising a pick-and-place machine, thepick-and-place machine including a first tool to position the die on thesubstrate and a second tool to position the flame source relative to thedie.
 23. The system of claim 22, wherein the pick-and-place machinefurther includes a third tool having a heat shield to position aroundthe die.
 24. A method comprising: placing a die on a substrate, the diehaving a front side and an opposing back side, the die having a numberof leads extending from the front side, each of the leads mating with acorresponding one of a number of lands on the substrate; and applying aheat source to the die back side to heat the die and bond each die leadto the corresponding one land on the substrate, the heat source selectedfrom a group consisting of superheated air, plasma, and steam.
 25. Themethod of claim 24, wherein each of the die leads comprises a solderbump.
 26. The method of claim 24, wherein the heat source is applied fora duration of up to approximately 2 seconds.
 27. The method of claim 24,wherein an absorbed heat flux at the die back side is at leastapproximately 40 W/cm².
 28. The method of claim 24, further comprisingplacing a heat shield around the die.